This invention relates to an optical inspection method and an optical inspection apparatus for detecting a contaminant particle or a defect of a sample to be inspected, such as a thin film substrate, a semiconductor substrate or a photomask by radiating light thereon, or in particular to an optical inspection method and an optical inspection apparatus having a sensitivity or a throughput improved as compared with the conventional method and apparatus.
In the production line of a semiconductor substrate or a thin film substrate, the inspection is conducted for contaminant particles attached on the surface of the semiconductor substrate or the thin film substrate to monitor the dust generation in the manufacturing equipment. In the semiconductor substrate before forming a circuit pattern, for example, the detection of fine contaminant particles or micro defects on the order of not more than several tens of nm on the surface thereof is required. A conventional technique for detecting fine defects on the surface of a sample such as a semiconductor substrate is described in, for example, U.S. Pat. No. 5,798,829, in which a focused laser light flux is fixedly radiated on the surface of the semiconductor substrate (the illuminated area formed on the semiconductor substrate surface by the laser light flux radiated is called an illumination spot), and the scattered light generated from a contaminant particle, if any, attached on the semiconductor substrate is detected while rotating and translating the semiconductor substrate so that the whole surface of the semiconductor substrate is inspected for a contaminant particle or defect. For detecting the scattered light, an ellipsoidal mirror is used. The detection position on the semiconductor substrate is defined a primary focus position of the ellipse and the light-receiving surface of a photodetector is arranged at a secondary focus position. In this way, the scattered light generated from a contaminant particle is focused at a wide solid angle to detect even a fine contaminant particle. In this conventional technique, only one laser light flux for illuminating the semiconductor substrate corresponds to one incident angle, and only one illumination spot is formed on the semiconductor substrate surface by the particular laser light flux.
Another conventional technique is described in, for example, JP-A-2001-255278, in which a condenser lens and a photodetector are arranged at combined positions of a plurality of elevation angles and azimuthal angles with respect to the surface of the semiconductor substrate, and the scattered light focused by each condenser lens is detected by the photodetector, so that a fine contaminant particle can be detected in an advantageous direction conforming with the three-dimensional radiation distribution characteristic of the scattered light from the particular fine contaminant particle. In this prior art, although two laser light fluxes for illuminating the semiconductor substrate exist for oblique and normal illumination, only one laser light flux corresponds to one incident angle and also only one illumination spot is formed on the semiconductor substrate surface by the particular laser light flux.
With the semiconductor substrate (semiconductor wafer), the thin film substrate and the photomask, the size of the contaminant particle or defect requiring detection is sharply reduced with the increase in package density. In the case where the particle size of the contaminant particle is so small as to follow the Rayleigh scattering, the scatter signal amount S obtained by the photodetector detecting the scattered light from the contaminant particle to be detected on a flat, smooth sample surface is generally proportional to the value of the right side of the following equation:S∝(illuminance of illumination beam)×(size of contaminant particle to the power of 6)×(illumination wavelength to the power of −4)×(collection efficiency of scattered light detection optics)×(duration of scattered light)×(quantum efficiency of photodetector)×(gain of photodetector)where the noise level N for detection is generally substantially proportional to the value of the right side of the following equation:Square root of N∝(illuminance of illumination beam)×(area of illumination spot)×(scattering efficiency of sample surface)
The following factors, therefore, have so far been well known to improve the detection sensitivity of contaminant particles or defects:
(1) The illuminance of the illumination beam in the illumination spot is increased to increase the strength of the scattered light.
(2) The wavelength of the illumination beam is shortened to increase the strength of the scattered light.
(3) The numerical aperture of the focusing optics is increased to increase the efficiency of focusing the scattered light.
(4) The performance of the photodetector such as quantum efficiency and S/N is improved.
(5) The background scattering is reduced by reducing the area of the illumination spot.
(6) The primary scanning rate of the sample stage is reduced to lengthen the time for the contaminant particle or defect to pass through the illumination spot.
(7) The diameter of the illumination spot along the primary scanning direction is increased to lengthen the time for a contaminant particle or defect to pass through the illumination spot.
Under the circumstances, however, it is not easy to improve the sensitivity even if these measures are taken, for the reasons described below.
(1) An increased illuminance of the illumination beam causes the surface of the sample to absorb the energy of the illumination beam and increases the surface temperature of the sample thereby increasing the risk of thermal damage to the sample.
(2) The wavelength of the available light source having an output suitable for detecting a contaminant particle or defect is limited and cannot be shortened to less than a certain limit.
(3) The collection efficiency of the scattered light emitted from contaminant particles or defects fails to reach more than 100%. The figure is generally about 50% in the prior art, and this value cannot be doubled in the future.
(4) In the prior art, the quantum efficiency and S/N of the photomultiplier tube used as a photodetector suitable for detecting weak scattered light have almost reached a theoretical limit and a further improvement thereof is not expected.
(5) The area of the illumination spot can be effectively reduced at the sacrifice of a lengthened time required to inspect the whole surface of the sample to be inspected.
(6) A lower primary scanning rate, like in (5), leads to the disadvantage of a longer time required to inspect the whole surface of the sample.
(7) The mere increase in the diameter of the illumination spot along the primary scanning direction is not effective as it is offset by the increased background scattering due to the increased area of the illumination spot, while a decreased diameter of the illumination spot along the direction orthogonal to the primary scanning direction to prevent the illumination spot area from increasing, on the other hand, like in (5), disadvantageously lengthens the time required for inspection of the whole surface of the sample.